The present invention generally relates to methods of growing compound semiconductor epitaxial layer by an atomic layer epitaxy, and more particularly to a method of growing atomic layers of a compound semiconductor on a substrate by use of the atomic layer epitaxy.
A atomic layer epitaxy (hereinafter simply referred to as an ALE) is a method of growing a compound semiconductor epitaxial layer. When growing the compound semiconductor epitaxial layer, molecules including positive ions (cations) and molecules including negative ions (anions) are alternately supplied to a substrate so as to grow epitaxial layers in atomic monolayers. In order for an ideal ALE to be carried out, a source material used must have a self-limiting effect so that the material is adsorped on the substrate up to a certain quantity, but no further adsorption of that material occurs when that certain quantity is reached.
Conventionally, there is an apparatus shown in FIG. 1 for growing a compound semiconductor epitaxial layer by ALE. An example of this type of apparatus is disclosed in Japanese Laid-Open Patent Application No. 62-270493. This type of apparatus utilizes an ALE deposition chamber which was originally used for a chemical vapor deposition (CVD). In FIG.1, the apparatus generally has a reactor 1, a susceptor 2, a gas introducing part 4, a gate valve 5, a gas exhaust part 6, and a high-frequency coil 7 wound around the outer periphery of the reactor 1. A substrate 3 is placed on the susceptor 2.
When growing a layer by ALE using zinc sulfide (ZnS) as the compound source material, a zinc chloride (ZnCl.sub.2) gas is introduced from the gas introducing part 4 as a Zn source material for a predetermined time to fill the inside of the reactor 1. Hence, as shown schematically in FIG. 2A, Zn compound molecules, including chlorine radicals, are adsorped on the substrate 3. In FIG. 2A through 2C, a plain circular mark indicates a Zn atom, a rectangular mark indicates a Cl atom, a circular mark with a dot pattern indicates a S atom, and a triangular mark indicates a hydrogen radical.
When the adsorped Zn compound molecules cover the surface of the substrate 3, a self-limiting effect occurs. Accordingly, no further Zn compound molecules are adsorped and the growth of the Zn compound layer is stopped.
Next, a hydrogen sulfide (H.sub.2 S) gas is introduced from the gas introducing part 4 for a predetermined time. Hence, as shown schematically in FIG. 2B, S compound molecules including negative ions made up of hydrogen radicals and S atoms are adsorped on the Zn compound molecule layer, and the hydrogen radicals of the S compound molecules and the chlorine radicals of the Zn compound molecules react. As a result, after a predetermined time, a S atomic layer 10 is formed on a Zn atomic layer 9 as shown schematically in FIG. 2C. Similarly thereafter, the Zn atomic layer and the S atomic layer are alternately grown until a ZnS layer is formed to a desired thickness.
Therefore, the self-limiting effect is conventionally obtained by supplying a compound source material having the self-limiting effect on a substrate surface.
However, compound source materials having such a self-limiting effect hardly exist in reality. For example, when growing gallium arsenide (GaAs) by ALE, it is possible to realize a state similar to that shown in FIG. 2A by using a Ga source material such as trimethylgallium (TMGa: (CH.sub.3).sub.3 Ga) and keeping the substrate temperature at a sufficiently low temperature, but a reaction time is very slow, at the growth surface such as that shown in FIG. 2B, realized by use of any As compound source material.
And, when growing the GaAs molecular layer on the substrate, the substrate temperature should be set to a sufficiently high temperature so as to decompose the Ga compound molecules adsorped on the substrate into Ga atoms by the thermal energy of the substrate and then supply an As compound source material such as arsine (AsH.sub.3) into the reactor 1.
But in this case, a gas stagnant layer 8 exists above the substrate 3 as shown in FIG. 1, and the source material gas must pass through the gas stagnant layer 8 in order to reach the substrate 3. Consequently, as shown schematically in FIG. 3, Ga molecules 11 made up of Ga atoms lla and methyl radicals llb are decomposed while passing through the gas stagnant layer 8 before reaching the substrate 3, and the decomposed Ga atoms lla are adsorped on the substrate 3. Because the Ga atoms lla do not have the self-limiting effect, there is a problem in that the Ga atomic layer is not self-limited to a predetermined thickness.
FIG. 4 shows a relationship between a thickness of grown GaAs layer per material supply cycle and a flow (supply) time of TMGa gas at a substrate temperature T.sub.sub of 500.degree. C. when the conventional method is employed. The flow time of the AsH.sub.3 gas was kept constant. The thickness of the grown GaAs layer increases as the flow time t of the TMGa gas increases, and it is extremely difficult to control the thickness of the grown GaAs layer to a desired thickness of 2.83 .ANG. corresponding to one molecular layer made up of one Ga atomic layer and one As atomic layer.
The decomposition of the Ga molecules which occurs when the TMGa gas passes through the gas stagnant layer 8 can be suppressed by reducing the substrate temperature T.sub.sub because the temperature of the gas stagnant layer 8 is reduced when the substrate temperature T.sub.sub is reduced. FIG. 5 shows a relationship between the thickness of grown GaAs layer per material supply cycle and a flow (supply) time of TMGa gas at a substrate temperature T.sub.sub of 400.degree. C. when the conventional method is employed. The flow time of the AsH.sub.3 gas is kept constant. One molecular layer having the desired thickness of 2.83 .ANG. is obtained when the flow time t of the TMGa gas is greater than a certain value.
But in this case, the reaction of the TMGa gas and the AsH.sub.3 gas, which occurs at the substrate surface, is slow because of the low substrate temperature T.sub.sub, and the time it takes to grow the GaAs molecular layer in FIG. 5 is long compared to the case shown in FIG. 4. In addition, when the growth of the GaAs molecular layer is slow, there is a problem in that the crystal state of the grown GaAs molecular layer is unsatisfactory since unwanted impurities in the GaAs molecular layer increase with the growth time.